AMP form problems (very slow)

Question

When clicking send the form is taking upto 30 seconds to send.

The form is working, it does send eventually.

If I inspect the page and view network to try and see what is going on it sends instantly as expected. When viewing the page normally it is really slow.

I have no idea what would be causing this? Does anyone else have a similar issue?

I have contacted my hosting company to confirm the issue is my end rather than a server issue.

HTML

<form method="post" action-xhr="send-form.php">
Your name:<br>
           <input type="text" name="namee" required  />
           Your telephone number:<br>
            <input type="text" name="phone" required />
            Your email address:<br>
           <input type="text" name="email" required />
           Your message:<br>
           <textarea name="message"></textarea>

            <input type="submit" class="imgcenter" value="" background-image: url(https://www.example.co.uk/img/send.png);"/>
</form>

PHP

<?php
if( !empty($_POST) ){

    $email = $_POST['email'];
    $namee = $_POST['namee'];
    $phone = $_POST['phone'];
    $message = $_POST['message'];

    $formcontent=" From: $email" . "$namee" . "$phone" . "$message";
    $recipient = "[email protected]";
    $subject = "Enquiry";
    $mailheader = "From: $email \r\n";
    mail($recipient, $subject, $formcontent, $mailheader) or die("Error!");

    $domain_url = (isset($_SERVER['HTTPS']) ? "https" : "http") . "://$_SERVER[HTTP_HOST]";
    $redirect_url = 'https://www.example.co.uk/thanks.html'; //-->MUST BE 'https://';

    header("Content-type: application/json");
    header("Access-Control-Allow-Credentials: true");
    header("Access-Control-Allow-Origin: *.ampproject.org");
    header("AMP-Access-Control-Allow-Source-Origin: ".$domain_url);


        if( empty($redirect_url) ){
            header("Access-Control-Expose-Headers: AMP-Access-Control-Allow-Source-Origin");
        }else{
            header("AMP-Redirect-To: ".$redirect_url);
            header("Access-Control-Expose-Headers: AMP-Redirect-To, AMP-Access-Control-Allow-Source-Origin");
        }     
}
?>